why is the display modifier button in blender 2.8 purple?
Purple indicates a driver.

For the most part, (unless you are using some whaky theme) purple in a field indicates a driver has been added.

To remove, right click over field and choose delete driver. (adding driver is most likely how you got there in the first place)


Oops added driver to diff prop from Q, same deal though

Note

In Q screenshot you have the apply all and viewport vis buttons etc above modifiers, which I assume is from an addon.  Possibly this is using a driver on the modifier visibility switch